1. Field of the Invention
This invention relates to an antireflective member used in various optical devices such as a spectroscopic analysis, optical electronics, optical communications and lighting equipment and suitably used to prevent light having wavelengths in the visible range from reflecting.
2. Description of Prior Art
Conventional optical elements using transparent substrates such as glasses and plastics are subjected to a surface treatment, for example an antireflective film provided on a light incident surface of the substrate, in order to reduce the amount of light reflected from the surface. A multilayered film with thin dielectric films deposited is well known as the antireflective film for light of wavelengths in the visible range. Such a multilayered film is formed by growing thin films of metal oxides etc. on a surface of a transparent substrate by means of a vacuum evaporation method or the like.
Another technique of the surface treatment to prevent reflection is to roughen the surface of the optical element finely and densely. When light passes through the surface of the optical element on which roughness is regularly provided, generally light diffraction occurs, thereby significantly reducing straight component of the transmitted light. However, roughness whose pitch is shorter than a wavelength of light transmitting therethrough may not cause the light diffraction. If the roughness is made, for example, in a rectangular shape as described later, excellent antireflective effect can be obtained for light having a single wavelength corresponding to the pitch, depth or the like of the roughness.
If the roughness is in the shape of peak and valley rather than rectangular, in other words, if the roughness takes the shape of a pyramid in a manner which continually changes a ratio between a volume of the optical element member side and a volume of the air side, an excellent antireflective effect can be obtained for even light having a wide wavelength range.
An example of a method to form such a roughness is cited in JP 88001/1993A. In the disclosure, an alcohol solution of ethyl silicate blended with particles of silicon oxide or aluminum oxide is coated over a surface of the required optical element. A film with roughness can be obtained after removing the particles.
JP 96902/1987A discloses another method. A surface of a molding tool is worked with the dense sawtooth shapes (also included are sawtooth shapes with rounded edge) at the prescribed depth from the depth of ⅓ wavelength of visible rays up to 1/50 and the plastic is molded by using the finished molding tool.
In the above method to form the antireflective film consisting of the dielectric thin films on the surface of the optical element, however, it is difficult to obtain the antireflective effect for light having a wavelength range if the antireflective film is made with a few dielectric thin films. Many dielectric thin films are needed to obtain the antireflective effect for light in a wide wavelength range. Also more dielectric thin films must be deposited to prevent reflectance from changing depending on an incident angle of light. Hence ten to twenty or more layers of the dielectric thin film are needed to satisfy required performance.
A vacuum evaporation method to form the dielectric thin films causes variation of thickness of the films, resulting in instability of performance. In addition, after molding a substrate to be an optical element in one process, the method needs another process to form the dielectric thin film on the surface of the substrate, thereby reducing yields and increasing costs.
Basically antireflective films cancel out reflective light through the use of interference of light. Due to this, a refraction index of materials and a thickness of each dielectric thin film need to be controlled with high accuracy in the process of producing the dielectric thin film, thereby increasing costs with an increase in number of the dielectric thin film to be deposited. In addition, the increase in number of the dielectric thin film causes warps or the like in the substrate of the optical element, thereby reducing yields.
Spectral characteristics of thin films are influenced by properties of materials of the thin films, especially by a refraction index. The refraction index varies according to conditions of the film formation. Besides, limitations in materials of the film make it difficult to obtain ideal spectral characteristics.
On the other hand, an optical substrate with protrusions in a pyramidal shape thereon would be effective. In order to obtain the antireflective effect for light having a wider wavelength range, it is preferable that an aspect ratio, which is a ratio of height to pitch P of the protrusions, is large. In a structure disclosed in JP 88001/1993A, however, it is difficult in principle to form pyramidal shapes with the aspect ratio of not less than one.
Also in a structure disclosed in JP 96902/1987A, there is a possibility that incident light is reflected diffusely because of irregular shapes of the protrusions, resulting in low efficiency.
JP 272505/2001A proposes a method for forming spindle shapes on a surface of an optical element to solve the above problems. A metallic mask is formed on the optical element in a dot array and the optical element is subjected to reactive ion etching. At this time, the optical element is etched until the metallic mask vanishes after the gradual reduction of the diameter of the mask.
Although a surface treatment method of which the spindle shape with a large aspect ratio is formed on the optical element is proposed in JP 272505/2001A, the process, forming the metallic mask of chromium (Cr), aluminum (Al) or the like on the optical element and subjecting the optical element to reactive ion etching until the metallic mask vanishes, becomes complicated with increase in cost.
The present invention has an object to provide an antireflective member with stability and a low reflectance at low cost by performing an antireflective treatment to the substrate in the molding process of the substrate.